The present invention relates to a spectrum analyzer that repeats to sweep frequencies of a received signal, logarithmically amplifies the received signal, detects the amplified output, and converts the detected output into a digital signal value to display the spectrum of the received signal. More particularly, the present invention relates to an apparatus and a method for displaying a power spectrum of the received signal.
FIG. 1 shows a conventional spectrum analyzer. An input signal from an input terminal 11 is inputted to a frequency converter 12, where the input signal is frequency-mixed with a local signal from a local oscillator 13 to be converted into an intermediate frequency signal. This intermediate frequency signal is passed through an intermediate frequency bad-pass filter 14. The output of the filter 14 is logarithmically amplified by a logarithmic amplifier 15, and its dynamic range is compressed. The logarithmically amplified output is envelope-detected by a detector 16. The detected output is sampled by an AD converter 17 at a constant period, and each sample is converted into a digital value. This digital value is stored in a memory 18.
On the other hand, a central frequency of the signal to be received, a frequency band width, a frequency-sweep speed, a reference level and the like are inputted to the spectrum analyzer from input means 19. In accordance with those inputted data, a CPU (a central processing unit) reads out a program stored in a program memory 22, decodes and executes the program, and sets an oscillation central frequency of the local oscillator 13. In addition, the CPU controls a sweep signal generator 20 then the sweep signal generator 20 sweep-controls the oscillation frequency of the local oscillator 13 so that receivable frequencies of the input signal from the input terminal 11 can repeatedly be swept.
In the conventional trace-average displaying method, digital values Pin (dBm) obtained in the repeated frequency-sweeps for each of display points (pixels) i (i=1,2, . . . , N) on a frequency axis (usually lateral axis) of a display device 24 are read out from the memory 18, and the average value of those digital values Pin is obtained using a following equation by an average value calculating part 25.       P    iavg    =            1      M        ⁢                  ∑                  n          =          1                M            ⁢                        P                      i            ⁢                          xe2x80x83                        ⁢            n                          ⁡                  (          dBm          )                    
Pin is a power value obtained by nth sweep for ith display point, and the unit thereof is dBm for representing 1 mW to be 0 (zero) dB. In this manner, an average power Piavg for each display point i obtained by M sweeps is read out from an average value memory 26 and is displayed by the display device 24 under control of a display control part 23. For example as shown in FIG. 2, the average values Piavg are displayed on the longitudinal axis on a screen of the display device 24 for the respective display points 1xe2x88x92N on the lateral axis (frequency axis).
In this trace-average displaying method, an averaging process of the signal power values Pi (dBm) for each display point is performed. Therefore, this method is effective for averaging noises added on top of the signal. However, the measured values Pi for each display point are values each having been logarithmically compressed by the logarithmic amplifier 15, and an arithmetic average of those logarithmic values is simply calculated. For this reason, the calculated value of the arithmetic average is not a correct average of the power of the input signal for the display point (frequency) i. That is, the trace-average displaying method does not display a correct average power distribution.
A digital-modulated signal has similarities to a white noise, and its amplitude probability density function depends on characteristics of the modulation type and the base-band filter modulation bit. The function for performing a channel power measurement provided in the conventional spectrum analyzer is described, for example, in the article entitled xe2x80x9cMeasure Adjacent-Channel Power With A Spectrum Analyzerxe2x80x9d written by J. Wolf and B. Buxton in a magazine xe2x80x9cMicrowave and RFxe2x80x9d, June 1997. This function measures a power for each display point and obtains a channel power by the following equation.   10  ⁢      xe2x80x83    ⁢      log    ⁡          [                        CHBW                      RBW            xc3x97                          k              n                                      xc3x97                  1                                    n              2                        -                          n              1                                      xc3x97                                            ∑              n                        1                          n              2                                ⁢                      10                                                            P                  i                                ⁡                                  (                  dBm                  )                                            10                                          ]        ⁢      (    dBm    )  
In this case, n1 and n2 are display point numbers of both sides of the channel, CHBW is the channel band width, RBW is the resolution band width of the spectrum analyzer, Pi is a level of ith display point (dBm), and k is a correction coefficient for the resolution band width (RBWxc3x97kn=power band width).
That is, measured values Pi (dBm) for each display point are converted into true values 10Pi/10 (mW) each having unit of watt (mW), and an average of the true values for each display point in the channel band is calculated. Further, power per channel band is obtained using the average value, and the logarithmic value of the power is displayed. In this case, the power of the entire channel band can be obtained, but the power of each spectrum or the power density of each display point cannot be obtained. In addition, a spectrum display of the power cannot also be performed.
Furthermore, it is described in the aforementioned magazine that the following equation is applied to the detected output voltage Vvideo of the spectrum analyzer to obtain the power of each display point.                               V                      rm            ⁢                          xe2x80x83                        ⁢            s                          =                                            1              T                        ⁢                                          ∫                0                T                            ⁢                                                V                  video                  2                                ⁢                                  ⅆ                  t                                                                                        (        1        )            
In this case, T is a power measuring time, and VVideo is an output voltage of an envelope detector.
In this case, the detected output voltage VVideo must have a linear scale. That is, the amplifier 15 having a linear amplification characteristic instead of a logarithmic amplification characteristic is used. Therefore, in order to materialize a power measurement of an input signal having a large dynamic range, it is necessary to use an amplifier, a detector and an AD converter each having a linear characteristic and a large dynamic range. Consequently, the equipment becomes expensive. On the other hand, if a logarithmic amplifier is used to obtain a large dynamic range by a low cost equipment configuration, the integrated value of the equation (1) does not represent a correct power value. In addition, the arithmetic operation of the equation (1) is applied to the output of the AD converter 17 in FIG. 1. Therefore, it is necessary to provide a specialized integration circuit for performing this arithmetic operation, and to perform the arithmetic operation at sufficiently high speed through digital calculations. It is shown that stable results can be obtained, and more accurate results can be obtained by increasing the integration time T, namely by increasing the sweep time.
However, it is not described in the magazine as to whether the sweep operation of the input frequencies is stopped during the time when the power value of one display point is being obtained, namely during the integration time T, or the input frequencies are swept at a uniform speed and the integrated value of the detected output Vvideo for each time length T is assumed to be the power value of one display point. In either case, in order to obtain the measured result, namely the stable integrated value of power, it is necessary to make the integration time T of each display point a relatively large value. In order to obtain the measured results of one channel band, namely the power spectrum display, it takes relatively long time. In addition, it is necessary to specially provide an integration circuit for performing the digital calculations of the equation (1) at sufficiently high speed, and in order to obtain the stable result during the integration time T, a considerable number of samples are required. It is necessary to considerably increase the sampling rate of the AD converter 17 in FIG. 1, and hence the AD converter 17 becomes expensive.
It is an object of the present invention to provide a spectrum analyzer that can display a correct power spectrum of an input signal having a large dynamic range and can be constructed at low cost, and to provide a spectrum measuring method using the spectrum analyzer.
It is another object of the present invention to provide a spectrum analyzer that can immediately display a power spectrum of an input signal regardless of modulation mode and can stably display more correct power values as time passes, and to provide a spectrum measuring method using the spectrum analyzer.
According to the present invention, sweeping of received frequencies is repeated, the received signal is logarithmically amplified, the amplified output is detected, the detected output is converted into a digital signal value, the digital signal value (dBm) is converted into a power value of anti-logarithm in watt (mW) dimension for each display point within a width of the frequency sweep, the power values having been converted into anti-logarithms that are obtained at respective frequency sweeps are averaged for each display point, the average power value for each display point is converted into a logarithmic value, and those logarithmic average power values for respective display points are displayed as a spectrum display.